Lead weight with steel vs Al tanks concerns

[jfw432]

Been reading a lot lately and not really finding what I'm looking for. I got certified about 20 years ago and just getting back into it after a long break so trying to get all of my equipment and weighting squared away.

I've had a handful of dives lately in both salt and fresh water using both Al and steel tanks. So far, I've haven't had the opportunity to explore my neutral buoyancy on a near empty tank. The waves have either been way too big in salt water or I haven't drained a tank down far enough in fresh water to do the eye level float. My latest dives were in the FL springs and my buoyancy felt pretty good with 11 lbs of lead, a 5mm suit, and an AL80. Everything felt good and I could control my buoyancy with just my lungs and maybe a little in the BCD at the start of the dive between 5ft and 30ft and had between 2800 and 1000 psi in the tank. Deeper dives used more weight as I was experimenting but obviously could've had less weight.

Where I have some concerns is with a steel HP80 tank I have. I was playing with it for my wife who is pretty short and also want an HP100 for myself....probably. I started off with 6lbs on the HP80 and felt way overweighted. I was struggling with buoyancy control at all depths. I lowered this to 3lbs and still felt heavy. Finally at the end of the dive, I had 1800psi in the tank and dropped like a rock with no lead. I am strong enough to swim up from 30-40 ft with no air in my BCD but it is alarming how fast I dropped if I let all air out of the BCD since I don't really have much else I can get rid of. I carry some trauma shears, a small dive light, and a DSMB but those aren't exactly heavy.

Do you consider this unsafe to dive with the steel tank with no lead and a 5mm suit? Should I stick to aluminum tanks? Eventually, I will get to deeper dives where I feel like this might be a bigger concern if I had a BCD failure. Just trying to decide if steel tanks are even right for me. If I went to warmer fresh water, I'd be even less buoyant with a 3mm full suit or a shorty suit and I wouldn't have anymore weight to shed with a steel tank. I've been playing with the OptimalBuoyancy spreadsheet and the numbers seem to be in the ballpark at least.

 

[rick00001967]

hp steel tanks are quite heavy in the water. they can be nice in cold water when using a dry suit and heavy undies etc.
but if you had no lead and felt too heavy then i think you should avoid using this for obvious reasons.
you could try a regular steel tank if there are some avail. but it sounds like an allum will be just fine for the diving you are doing.
if you want some ditchable lead (some divers dont - lets not go there) the allum may be the only choice for you.

 

[LI-er]

Finally at the end of the dive, I had 1800psi in the tank and dropped like a rock with no lead.

Do you consider this unsafe to dive with the steel tank with no lead and a 5mm suit? Should I stick to aluminum tanks?

It's all about what you mean when you say you "dropped like a rock".

Being slightly negative and sinking while wearing steel tanks and no lead is very common, happens to me all the time. But if you're sinking rapidly and you need to compensate by dumping loads of air into your BCD then that's a dealbreaker on the steels.

By the way, the buoyancy of steel tanks doesn't change significantly as the pressure drops. So how much you had left in the tank is irrelevant.

 

[johndiver999]

It's all about what you mean when you say you "dropped like a rock".

Being slightly negative and sinking while wearing steel tanks and no lead is very common, happens to me all the time. But if you're sinking rapidly and you need to compensate by dumping loads of air into your BCD then that's a dealbreaker on the steels.

By the way, the buoyancy of steel tanks doesn't change significantly as the pressure drops. So how much you had left in the tank is irrelevant.

This last sentence is incorrect. Tanks change buoyancy as a function of the air consumed, rather than the type of tank or its composition (steel, Al or CF).

The OP raises a big issue. Those tiny little, hp steel 80's are very negative. They may be too negative for some small people, who are not fat and are not wearing a thick suit, particularly in freshwater.

In general, you do not want to be any heavier when wearing a steel tank or an aluminum one. The potential benefit of an aluminum tank is that the diver generally has to take ditchable lead and some portion of this can be dropped when a thick suit is used and the diver is deep. If a similar thing happens with a steel tank and there is no ditchable lead, then you really should be strong enough to swim it up without a BC.

I often dive with a big steel tank and some times with zero ditchable lead (actually no lead) and I am heavier than I really want on the bottom, when I dump all the air from the BC, but I can swim it up, as long as I have air.

If you or your wife feel comfortable with no ditchable lead and can swim the unit up with no air in the BC, then the steel tank should be ok, even if it makes you a little nervous. The OP is smart to question this and should investigate the situation on their own, with the precise set of conditions that they will be presented with.

A hp steel 80 in freshwater for a small women in a shorty wetsuit and a skin, is probably not a good idea.

The same lady wearing a full 5mm suit in the ocean and she might find that tank wonderful.

The amount of fat a person has on their body, has a lot to do with it as well.

 

[inquis]

Finally at the end of the dive, I had 1800psi in the [HP80] tank and dropped like a rock with no lead.

At that pressure (44 cuft or 3.5 lb of air), you would ideally be negatively buoyant by about 2.5 lb if your BCD were empty and you exhaled normally. You could become neutral with about 1/3 of a full breath. Dropping "like a rock" suggests you were significantly more negatively buoyant.

You mentioned a "BCD". By chance, is this a stainless steel backplate & wing? Switching to an aluminum plate could increase buoyancy by 3-5 lbs (depending on designs).

Also, not all steel tanks are the same. Worthington steels, are usually very negative, while Fabers are less so.

 

[Badger7]

By the way, the buoyancy of steel tanks doesn't change significantly as the pressure drops

I'd like to gently correct this myth. The bouyancy of both aluminum and steel cylinders increases at the exact same rate as gas is consumed. 80 cubic feet of air weighs a little over 6 pounds. If you breath 80 cubic feet of gas from an aluminum AL80 or a steel HP80, either cylinder is going be 6 pounds _more_ bouyant. Change in bouyancy will be identical.

Depending on other factors such as the bouyancy of your exposure protection and how much lead you're wearing, breathing down a steel or aluminum tank may or may not make your entire kit positively bouyant with empty tanks. Regardless if you start out with a total bouyancy of -12 pounds, breathe 80cuft of gas, you'll be -6 pounds bouyant (-12 + 6 = -6). If you start out -3 pounds bouyant and you breathe down 80cuft of gas, you'll be +3 pounds bouyant and fighting to hold your safety stop (-3 + 6 = +3). This math does not change at all based on aluminum vs steel cylinders.

The material of the cylinder has zero bearing on the weight of the gas inside. Yes, steel tanks are heavier than aluminum tanks, full or empty, no question, but the bouyancy increase for every cubic foot of gas consumed is identical regardless. Yes, an empty aluminum tank is typically positively bouyant, and an empty steel tank is typically negative or neutrally bouyant, but the CHANGE in bouyancy is equivalent for any given gas volume consumption.

how much you had left in the tank is irrelevant.

Amount of gas consumed is pretty much the only thing that is relevant to a change in bouyancy. Literally the only other factor (and to a very small degree) is depth, as it compresses materials like neoprene, decreasing bouyancy with depth and increasing slightly on ascent.

 

[passeparici]

By the way, the buoyancy of steel tanks doesn't change significantly as the pressure drops. So how much you had left in the tank is irrelevant.

This is incorrect, the buoyancy of a steel tank or an al tank change the same way as it is the volume of air you use that impacts the buoyancy.

To have an idea of the weight you should take for your dive, when you got rid of all the air in your BCD at the beginning of a dive, you should start to sink at moderate speed (certainly not "drop like a rock"). Most of the time, if you start like that, it should be ok at the end of the dive (no air in your BCD or just a tiny bit you should be able to stay at your safety stop). If you start your dives and "drop like a rock" without any additional weight it is unsafe. Use another tank. It is difficult to give a straight answer to your questions as many things get into play : your suit, the weight and buoyancy of your BCD (I have a Hydropro and a wing and there is almost 3 kg difference between those 2), the weight of your accessories, etc.

 

[KenE]

The surface wave condition should not affect an accurate weight check. An accurate weight check should be done at 15-20 ft with a tank at reserve pressure. If you can safely hold your safety stop then the weight is accurate. I suggest doing this in 16-18 ft of water and start removing lead and placing it on the sea bottom. When you can no longer stay down then pickup one pound and that is your proper weight. I suggest doing this for each tank you use and write it down. Then you will have your proper weight for that rig. As you make changes to your rig you will need to make changes to your weight, but it will be much easier. Adding or losing significant body weight will also affect the amount required. Diving a pound heavy is not bad, but anything more and your consumption will increase. I hope this helps get it worked out.

 

[jfw432]

Ok so I'll try to clarify a few things... First, I just have a Mares jacket style BCD. Mine provides 45lb of lift while my wife's only provides 31lbs since it's so much smaller. Both are more than enough providing they are working properly which is of course is where the concern comes in.

"Dropping like a rock" is ambiguous for sure. To put it plainly, as I was experimenting with no air in the BCD, I would have to kick once or twice every time I needed to equalize before getting too deep after the previous equalization and I only went down to 15-20 ft.

This steel tank is a Faber HP80.

 

[Badger7]

"Dropping like a rock" is ambiguous for sure. To put it plainly, as I was experimenting with no air in the BCD, I would have to kick once or twice every time I needed to equalize before getting too deep after the previous equalization and I only went down to 15-20 ft.

If you're wearing light exposure protection like shorty or a 5mm, and you want ditchable lead in case of a BCD bladder failure, you might be better off diving an aluminum tank. Only way to know is a proper weight check, and kicking up is too subjective. If you completely empty your BCD, tank at 500psi, and you're still more negative at 15ft than full lungs will counteract, you're too heavy.